AASHTO Pavement Design 1986

AASHTO@ GUIDEFOR

DESIGN OF PAVEMENT STRUCTURES1986

Published by theAmerican Association of State Highway

and Transportation Officials

It44 N. Capitol Street, N. W., Suite 225Washington, D. C. 2(X)01

@Copyright, 1986, by the American Association of State Highwayand Transportation 0fficials. All Rights Reserved. Printed in theUnited States of America. This book, or parts thereof, may not bereproduced in any form without written permission of thepublishers.

{ffi,19l-4

Design Requirements

section), it is strongly recommended that the designer

use mean (average) values rather than "conservative

estimates"for each of the design inputs required by theprocedures. This is important since the equations were

developed using mean values and actual variations.

Thus, the design er must use meon values and standard

deviations associated with his conditions.

2.T DESIGN VARIABLES

2.1.1Time Constraints

This section involves the selection of performanceand analysis period inputs which affect (or constrain)pavement design from the dimension of t ime.Consideration of these constraints is required for bothhighway and low-volume road design. Time constraintspermit the designer to select from strategies rangingfrom the initial structure lasting the entire analysisperiod (i.e., performance period equals the analysisperiod) to stage construction with an initial structureand planned overlays.

Perlormance Period. This refers to the period oftime that an initial pavement structure will last beforeit needs rehabilitation. It also refers to the performancetime between rehabilitation operations. In the designprocedures presented in this Guide, the performanceperiod is equivalent to the time elapsed as a new,reconstructed, or rehabilitated structure deterioratesfrom its initial serviceability to its terminal service-ability. For the performance period, the designer mustselect minimum and maximum bounds that are estab.lished by agency experience and policy. It is importantto note that, in actual practice, the performance periodcan be significantly affected by the type and level ofmaintenance applied. The predicted performanceinherent in this procedure is based on the maintenancepractlces at the AASHO Road'l 'est.

The minimum performance period is the shortestamount of time a given stage should last. Forexample,it may be desirable that the initial pavement structurelast at least l0 years before some major rehabilitationoperation is performed. The limit may be controlledby such factors as the public's perception of how long a"new" surface should last, the funds available forinitial construction, life-cycle cost, and other engi-neering considerations.

The moximum performance period is the maximumpractical amount of time that the user can expect froma given stage. For example, experience has shown in

II-7

areas that pavements originally designed to last 20years required some type of rehabilitation or resur-facing within l5 years after initial construction. Thislimiting time period may be the result of PSI loss dueto environmental factors, disintegration of surface,etc. The selection of longer time periods than can beachieved in the field will result in unrealistic designs.Thus, if life-cycle costs are to be considered accurately,it is important to give some consideration to themaximum practical performance period of a givenpavement type.

Analysis Period. This refers to the period of timefor which the analysis is to be conducted, i.e., thelength of time that any design strategy must cover. Theanalysis period is analogous to the term design lifeused by designers in the past. Because of the con-sideration of the maximum performance period, itmay be necessary to consider and plan for stageconstruction (i.e., an initial pavement structure fol-lowed by one or more rehabilitation operations) toachieve the desired analysis period.

In the past, pavements were typically designed andanalyzed for a 2O-year performance period, since theoriginal Interstate Highway Act in 1956 required thattraffic be considered through 1976. It is now recom-mended that consideration be given to longer analysisperiods, since these may be better suited for theevaluation of alternative long-term strategies based onlife-cycle costs. Consideration should be given toextending the analysis period to include one rehabili-tation. For high-volume urban freeways, longeranalysis periods may be considered. Following aregeneral guidelines:

HighwayConditions

Analysis Period(years)

High volume urbanHigh volume ruralLow volume pavedLow volume

aggregate surface

3 0 - 5 02 0 - 5 01 5 - 2 5l 0 - 2 0

2.1.2 Traffic

The design procedures for both highways and low-volume roads are all based on cumulative expectedl8-kip equivalent single axle loads (ESAL) during theanalysis period (Sra). The procedure for convertingmixed traffic into these l8-kip ESAL units ispresented in Part I and Appendix D of this Guide.Detailed equivalency values are given in Appendix D.

II.8

For any design situation in which the initial pavementstructure is expected to last, the analysis periodwithout any rehabilitation or resurfacing, all that isrequired is the total traffic over the analysis period. If,however, stage construction is considered, i.e.,rehabilitation or resurfacing is anticipated (due to lack

10 .o

Design of Pavement Structures

of initial funds, roadbed swelling, frost heave, etc.),then the user must prepare a graph of cumulativel8-kip ESAL traffic versus time, as illustrated inFigure 2. l. This will be used to separate the cumulativetraffic into the periods (stages) during which it isencountered.

0c

:

.9(U

orr,CL

.v.t

@

o.:(g

:Ef

(J

1 0

Time (years)1 5 20

Figure 2.1. Example plot of cumulative 18-kip ESAL traff ic versus t ime.

Design Requirements

The predicted traffic furnished by the planninggroup is generally the cumulative l8-kip ESAL axleapplications expected on the highway, whereas thedesigner requires the axle applications in the designlane. Thus, unless specifically furnished, the designermust factor the design traffic by direction and then bylanes (if more than two). The following equation maybe used to determine the traffic (*rs) in the designlane:

w l 8 = D o * D r x 0 , ,

where

DD = a directional distribution factor, ex-pressed as a ratio, that accounts for thedistribution of ESAL units by direction,e.9., east-west, north-south, etc.,

DL = a lane distribution factor, expressed as aratio, that accounts for distribution oftraffic when two or more lanes areavailable in one direction.

Afr,t = the cumulative two-directional l8-kip

ESAL units predicted for a specificsection of highway during the analysisperiod (from the planning group).

Although the Do factor is generally 0.5 (50 percent)for most roadways, there are instances where moreweight may be moving in one direction than the other.Thus, the side with heavier vehicles should be designedfor a greater number of ESAL units. Experience hasshown that Do may vary from 0.3 to 0.7, depending onwhich direction is "loaded" and which is "unloaded."

For the D, factor, the following table may be usedas a guide:

No. of Lanes lnEach Direction

Percent of l8-kipESAL

In Design Lane

t0080 - 1006 0 - 8 05 0 - 7 5

2.1.3 Reliability

Reliability concepts were introduced in Chapter4 ofPart I and are developed fully in Appendix EE of

II.9

Volume 2. Basically, it is a means of incorporatingsome degree of certainty into the design process toensure that the various design alternatives will last theanalysis period. The reliability design factor accountsfor chance variations in both traffic prediction (*rs)and the performance prediction (W,r), and thereforeprovides a predetermined level of assurance (R) thatpavement sections will survive the period for whichthey were designed.

Generally, as the volume of traffic, difficulty ofdiverting traffic, and public expectation of availabilityincreases, the risk of not performing to expectationsmust be minimized. This is accomplished by selectinghigher levels of reliability. Table 2.2 presents recom-mended levels of reliability for various functionalclassifications. Note that the higher levels correspondto the facilities which receive the most use, while thelowest level, 50 percent, corresponds to local roads.

As explained in Part I, Chapter 4, design-perfor-mance reliability is controlled through the use of areliability factor (Fn) that is multiplied times thedesign period traffic prediction (*rs) to producedesign applications (W,s) for the design equation. Fora given reliability level (R), the reliability factor is afunction of the overall standard deviation (So) thataccounts for both chance variation in the trafficprediction and normal variation in pavement perfor-mance prediction for a given Wrs.

It is important to note that by treating designuncertainty as a separate factor, the designer shouldno longer use "conservative"estimates for all the otherdesign input requirements. Rather than conservativevalues, the designer should use his best estimate of themean or average value for each input value. Theselected level of reliability and overall standard devia-tion will account for the combined effect of thevariation of all the design variables.

Application of the reliability concept requires thefollowing steps:

(l) Define the functional classification of thefacility and determine whether a rural orurban condition exists.

Select a reliability level from the range givenin Table 2.2. The greater the value ofreliability, the more pavement structurerequired.

A standard deviation (S) should be selectedthat is representative of local conditions.

I234

(2)

(3)

II- IO Design of Pavement Structures

T a b l e 2 . 2 . S u g g e s t e d l e v e l s o f r e l i a b i l i t y f o r v a r i o u s f u n c t i o n a l

c lassi f icat ions.

Recommended Level of Reliabil ityFunct ional

Classification Urban Rural

lnterstate and otherfreeways

PrincipalArterials

Collectors

Local

85

80

80

99.9

99

95

80

75

99.9

95

95

80

Note: Results based cin a survev of the AASHTO Pavement Design TaskForce

Values of So developed at the AASHORoad Test did not include traffic error.However, the performance prediction errordeveloped at the Road Test was .25 for rigidand .35 for flexible pavements. This cor-responds to a total standard deviation for

traffic of 0.35 and 0.45 for rigid and flexiblepavements, respectively.

2.1.4 Environmental Effects

The environment can affect pavement performancein several ways. Temperature and moisture changescan have an effect on the strength, durability, andload-carrying capacity of the pavement and roadbedmaterials. Another major environmental impact is thedirect effect roadbed swelling, pavement blowups,frost heave, disintegration, etc., can have on loss ofriding quality and serviceability. Additional effects,such as aging, drying, and overall material deteriorationdue to weathering,are considered in this Guide only interms of their inherent influence on the pavementperformance prediction models.

The actual treatment of the effects of seasonal

temperature and moisture changes on material

properties is discussed in Section 2.3, 'Material

Properties for Structural Design."This section provides

only the criteria necessary for quantifying the input

requirements for evaluating roadbed swelling andfrost heave. If either of these can lead to a significantloss in serviceability or ride quality during the analysisperiod, then it (they) should be considered in the

design analysis for all pavement structural types,except perhaps aggregate-surfaced roads. As service-ability-based models are developed for such factors aspavement blowups, then they may be added to thedesign procedure.

The objective of this step is to produce a graph of

serviceability loss versus time, such as that illustrated

in Figure 2.2. As described in Part I, the serviceability

loss due to environment must be added to that

resulting from cumulative axle loads. Figure 2.2

indicates that the environmental loss is a result of the

summation of losses from both swelling and frost

heave. The chart may be used to estimate the service-

ability loss at intermediate periods, e.g., at l3 years the

loss is 0.73. Obviously, if only swelling or only frost

heave is considered, there will be only one curve on thegraph. The environmental serviceability loss is evalu-

ated in detail in Appendix G, "Treatment of Roadbed

Swelling andlor Frost Heave in Design."

il-ilDesign Requirements

(0.73)

1 0 1 3

Time (years)

F igure2.2 . A conceptua t example o f the env i ronmenta l serv iceab i l i t y loss versus t ime graph tha t

may be developed tor a speci f ic locat ion.

I

r.r

LL

c

c

63

V)

o<n

J

.=-o

' t

a

=E

(J

Total Loss,

A PStsw,rx

Frost HeaveLoss,

A est,,

Swell ing Loss,

A RStr*

II- 12

2.2 PERFORMANCE CRITERIA

2.2.1 Serviceability

The serviceability of a pavement is defined as its

ability to serve the type of traffic (automobiles and

trucks) which use the facility. The primary measure of

serviceability is the Present Serviceability Index (PSI),

which ranges from 0 (impossible road) to 5 (perfect

road). The basic design philosophy of this Guide is the

serviceability-performance concept, which provides a

means of designing a pavement based on a specific

total traffic volume and a minimum level of service-

ability desired at the end of the performance period.

Selection of the lowest allowable PSI or terminal

serviceability index (pt) is based on the lowest index

that will be tolerated before rehabilitation, resur-

facing, or reconstruction becomes necessary. An index

of 2.5 or higher is suggested for design of major

highways and 2.0 for highways with lesser traffic

volumes. One criterion for identifying a minimum

level of serviceability may be established on the basis

of public acceptance. Following are general guidelines

for minimum levels of p, obtained from studies in

connection with the AASHO Road Test (l l):

TerminalServiceability

Level

Percent of PeopleStating

Unacceptable

3.02.52.0

For relatively minor highways where economics dictate

that the initial capital outlay be kept at a minimum, it

is suggested that this be accomplished by reducing the

design period or the total traffic volume, rather than

by designing for a terminal serviceability less than 2.0.

Since the time at which a given pavement structure

reaches its terminal serviceability depends on traffic

volume and the original or initial serviceability (p),

some consideration must also be given to the selection

of po. (It should be recognized that the po values

obseived at the AASHO Road Test were 4.2 for

flexible pavements and 4.5 for rigid pavements. )

Once po and pt are established, the following

equation should be applied to define the total change

in serviceability index:


The equation is applicable to flexible, rigid, and

aggr e gatvs urfaced road s.

2.2.2 Allowable Rutting

In this design guide, rutting is considered only as a

performance criterion for aggregate-surfaced roads.

Although rutting is a problem with asphalt concrete

surface pavements, no design model suitable for

incorporation into this Guide is available at this time.

It is important to note that the rut depth failure

predicted by the aggregate-surfaced road model does

not refer to simple surface rutting (which can be

corrected by normal blading operations), but to

serious rutting associated with deformation of the

pavement structure and roadbed support. The al-

lowable rut depth for an aggregate-surfaced road is

dependent on the average daily traffic. Typically,

allowable rut depths range from 1.0 to 2.0 inches for

aggr e gate-s u rfaced ro ad s.

2.2.3 Aggregate Loss

For aggregate-surfaced roads, an additional concern

is the aggregate loss due to traffic and erosion. When

aggregateloss occurs, the pavement structure becomes

thinner and the load-carrying capacity is reduced. This

reduction of the pavement structure thickness increases

the rate of surface deterioration.

To treat aggregate loss in the procedure, it is

necessary to estimate (l) the total thickness of aggregate

that will be lost during the design period, and (2) the

minimum thickness of aggregate that is required to

keep a maintainable working surface for the pavement

structure.

Unfortunately, there is very little information

available today to predict the rate of aggregate loss.

Below is an example of a prediction equation developed

with limited data on sections experiencing greater than

50 perccnt truck traffic (ts, to):

GL = 0.12+ 0.1223(LT)

where

GL = total aggregate loss in inches,

LT = number of loaded trucks in thousands.

A second equation, which was developed from a

recent study in Brazil on typical rural sections, can be

t 25585

A P S I = p o - p t

4 . { - 2 . Z , d

Design Requirements

employed by the user to determine the input for gravelloss (rs, to):

GL = ?{r"2;ili.0045lADr

+ 3380 6/ R

where

GL = aggregate loss, in inches, during theperiod of time being considered,

B = number of bladings duringthe period oftime being considered,

LADT = average daily traffic in design lane (forone-lane road use total traffic in bothdirections),

R = average radius of curves, in feet,

G = absolute value of grade, in percent.

Another equation, developed through a Britishstudy done in Kenya, is more applicable to areas wherethere is very little truck activity and thus the facility isprimarily used by cars. Since this equation (below) isfor annual gravel loss, the total gravel loss (GL) wouldbe estimated by multiplying by the number of years inthe performance period:

AGL = f' I {r'*sol'4+.2 + .lszr+ .0889R2r.88VC)

where

AGL = annual aggregate loss, in inches,

T = annual traffic volume in both directions,in thousands of vehicles,

R = annual rainfall, in inches,

VC = average percentage gradient of the road,

f = .037 for lateritic gravels,

= .043 for quartzitic gravels,

= .028 for volcanic gravels

= .059 for coral gravels.

It should be noted that there are serious drawbackswith all the equations shown here; therefore, whenever

II.I3

possible,local information about aggregate loss shouldbe used as input to the procedure.

2.3 MATERIAL PROPERTIES FORSTRUCTURAL DESIGN

2.3.1Effective Roadbed Soil Resilient Modulus

As discussed previously in this Part and Part I, thebasis for materials characterization in this Guide iselastic or resilient modulus. For roadbed materials,laboratory resilient modulus tests (AASHTO T274)should be performed on representative samples instress and moisture conditions simulating those of theprimary moisture seasons. Alternatively, the seasonalresilient modulus values may be determined by cor-relations with soil properties, i.e., clay content,moisture, PI, etc. The purpose of identifying seasonalmoduli is to quantify the relative damage a pavementis subjected to during each season of the year and treatit as part of the overall design. An effective roadbedsoil resilient modulus is then established which isequivalent to the combined effect of all the seasonalmodulus values. (The development of the procedurefor generating an effective roadbed soil resilientmodulus is presented in Appendix HH of Volume 2 ofthis Guide.)

The seasonal moisture conditions for which theroadbed soil samples should be tested are those whichresult in significantly different resilient moduli. Forexample, in a climate which is not subjected toextended sub-freezing temperatures, it would be im-portant to test for differences between the wet (rainy)

and dry seasons. It would probably not be necessary,howevcr, to toEt for the diffcrencc between sprilrg-wet

and fall-wet, unless there is significant diffcrcncc in theaverage rainfall during spring and fall. If operationsmake it difficult to test the roadbed soil for spring-thaw or winter-frozen conditions, then, for theseextreme cases, practical values of resilient moduli of20,009 to 50,000 psi may be used for frozen conditions,ind for spring-thaw conditions, the retained modulusmay be 20 to 30 percent of the normal modulus duringthe summer and fall periods.

Two different procedures for determining the sea-sonal variation of the modulus are offered as guidelines.

One method is to obtain a laboratory relationshipbetween resilient modulus and moisture content. Then,with an estimate of the in situ moisture content of thesoil beneath the pavement, the resilient modulus for

II. I4

each of the seasons may be estimated. An alternateprocedure is to back calculate the resilient modulus for

different seasons using the procedure described in Part

III usingdeflections measured on in-service pavements.

These may be used as adjustment factors to correct the

resilient modulus for a reference condition.

Besides defining the seasonal moduli, it is also

necessary to separate the year into the various com-ponent time intervals during which the differentmoduli are effective. In making this breakdown, it is

not necessary to specify a time interval of less than

one-half month for any given season. If it is notpossible to adequately estimate the season lengths, the

user may refer to Section 4.1.2, which provides criteria

suggested for the design of low-volume roads.

At this point, the length of the seasons and the

seasonal roadbed resilient moduli are all that is

required in terms of roadbed support for the design of

rigid pavements and aggregate-surfaced roads. For the

design of flexible pavements, however, the seasonaldata must be translated into the effective roadbed soil

resilient modulus described earlier. This is accom-plished with the aid of the chart in Figure 2.3. The

effective modulus is a weighted value that gives the

equivalent annual damage obtained by treating each

season independently in the performance equation

and summing the damage. It is important to note,

however, that the effective roadbed soil resilient

modulus determined from this chart applies only to

flexible pavements designed using the serviceability

criteria. It is not necessarily applicable to other

resilient modulus-based design procedures.

Since a mean value of resil ient modulus is used,

design sections with coefficient of variations greater

than 0.15 (within a season) should be subdivided into

smaller sections. For example, if the mean value of

resilient modulus is 10,000 psi, then approximately 99

percent of the data should be in a range of 5,500 to

14,500 psi.

The first step of this process is to enter the seasonal

moduli in their respective time periods. If the smallest

season is one-half month, then all seasons must be

defined in terms of half months and each of the boxes

must be filled. If the smallest season is one month, then

all seasons must be defined in terms of whole months

and only one box per month may be filled in.

The next step is to estimate the relative damage (ur)

values corresponding to each seasonal modulus. This

is done using the vertical scale or the corresponding

equation shown in Figure 2.3. For example, the


relative damage corresponding to a roadbed soil

resil ient modulus of 4,000 psi is 0.51 .

Next, the - u, values should all be added togetherand divided by the number of seasonal increments (12

or 24) to determine the average relative damage. The

effective roadbed soil resil ient modulus (Mp), then, isthe value corresponding to the average relative damageon the MR ur scale. Figure 2.4 provides an example ofthe application of the effective M* estimation process.

Again, it is emphasized that this effective M* value

should be used only for the design of flexible pavements

based on serviceabilitv criteria.

2.3.2 Effective Modulus of Subgrade Reaction

Like the effective roadbed soil resilient modulus for

flexible pavement design, an effective modulus of

subgrade reaction (k-value) will be developed for rigid

pavement design. Since the k-value is directly pro-

portional to roadbed soil resil ient modulus, the season

lengths and seasonal moduli developed in the previous

section wil l be used as input to the estimation of an

effective design k-value. But, because of the effects of

subbase characteristics on the effective design k-value,

its determination is included as a step in an iterative

design procedure (see Chapter 3). The development of

the actual procedure for generating this effective

modulus of subgrade reaction is presented in Appendix

HH of Volume 2 of this Guide.

2.3.3 Pavement Layer Materisls Characterization

Although there are many types of material properties

and laboratory test procedures for assessing the

strength of pavement structural materials, one has

been adopted as a basis for design in this Guide. If,

however, the user should have a better understanding

of the "layer coefficients" (see Section 2.3.5) that have

traditionally been used in the original AASHTO

flexible pavement design procedure, it is not essential

that the elastic moduli of thcsc matcrials be charac-

terized. In general, layer coeflicients derived from test

roads or satellite sections are preferred.

Elastic modulus is a fundamental engineeringproperty of any paving or roadbed material. For those

material types which are subject to significant per-

manent deformation under load, this property may

not reflect the material's behavior under load. Thus,

resilient modulus refers to the material's stress-strain

behavior under normal pavement loading conditions.

The strength of the material is important in addition to

r

Design Requirements

Average:

.50

'6ct

c)o=

G

g;J

f!o

ceahoCT

oU'Eo-oE'(ooG

J c l

o c f )

F 1E3 io xE o: F

- q x@

:l l

5

:t

c.oofg

UJ

5.0

10.0

1 3 . 0t

u t = u u l =

Effective Roadbed Soil Resi l ient Modulus, M^ (Rsi) = (corresponds to u/

Figure 2.3. Chart for estimating effective roadbod soil resilient modulus forflexible pavements designed using the serviceability criteria.

II-15

il-16 Design of Pavement Structures

5 0

1 0 0

1 3 . 0

Month

RoadbedSoi l

Modu lus ,

MR (Ps i )

Rela t ive

Damage,

u f

J a n .20,000 0.01

Feb .20,000 0.01

Mar2,W0 1 . 5 1

Apr4,000 0 . 5 1

May4,000 0 . 5 1

June7,000 0 1 3

J u l y7,000 0 . 1 3

Aug7,000 0 1 3

Sept7,000 0 . 1 3

Oct7,000 0 . 1 3

Nov4 fi)O 0 5 1

Dec20.000 0.01

S u m m a t i o n : X r , = 3.72

-oo

(v)

o=

6.

u;f

Eo

cg60)(r

o@

o)

(9o(r

f - N

o c ' )(I, c.i(otr: 6

do x

t o(v

a ' x.5U CC co

:F

0

c9

?(ofct

[!

Average: ; , = E' ' = 3 '72 : o '31

n 1 2

Effect ive Roadbed Soi l Res i l ient Modulus, M^ (Osi ) = 5,000 (corresponds to tr)

Figure 2.4. Chart for estimating effective roadbed soilresilient modulus forflexible pavements designed using the serviceability criteria.

i

11.48

other agency policy requirements. Generally, the layerthickness is rounded to the nearest inch, bui the use ofcontrolled grade slip form pavers may permity--nchincrements. In addition to the design i-value, otherinputs required by this rigid pavement design nomo-graph include:

(l) the estimated future traffic, Wl8 (Section2.1.2), for the performance perio'd,

'

(2) the reliability, R (Section 2.1.3),

(3) the overall standard deviation, So (Section2 . 1 . 3 ) ,

(4) design serviceability loss, A pSI =(Section 2.2.1),

^'rD' rrrDl = Pi - Pt

(5) concrete elastic modulus, E" (Sectio n2.3.3),

(6) concrete modulus of ruptur., S," (Section2.3.4),

(7) load transfer coefficient, J (Sectio n 2.4.2),and

(8) drainage coefficient, Ca (Section 2.4.1).

3.2.3 Stage Construction

Experience in some states has shown that there maybe a practical maximum performance period (Section2-l.l) associated with a given.igd puu.,n.nt which issubjected to some significant level of truck traffic. Toconsider analysis periods which are longer than thismaximum expected performance period o. to morerigorously consider the life-cycle costs of rigid pave-ment designs which are initially thinner, it is hecessaryto consider the stage construction (planned rehabili-tation) approach in the design process. It is alsoimportant to recognize the need to compound thereliability for each individual stage of the strategy. Forexample, if both stages of a two_stage strategy (aninitial PCC pavement with one overlay) have a 90percent reliability, the overarl reliability of the designstrategy would b..

9:9 x 0.9 or g l percent. Conversely,if an overall reliability of 95 per€nt is desired, theindividual reliability for each stage must be (0.95)L or97.5 percent.

To evaluate secondary stages of such stage construc-tion alternatives, the user should refer topart III ofthis Guide which addresses the design for pavementrehabilitation. That part not only provides a irocedure

Design of povement Structu

for designing overlays, but also provides criteria fcthe application of other rehabilitation methods thzmay be used to improve the serviceability and extenthe load-carrying capacity of the pavement. Thdesign example in Appendix I provides an illustratio:of the application of the stage construction approacJusing a planned future overlav.

3.2.4 Roadbed Swelling end Frost Heave

The approach to considering the effects of swellinland frost heave in rigid pavement design is almosridentical to that for flexibre pavements (Section 3. r.3)Thus, some of the discussion is repeated here.

Roadbed swelling and frost heave are both importantenvironmental considerations because of their potentialeffect on the rate of serviceability loss. Swelling refersto the localized volume changes that occur in exfansiveroadbed soils as they absorb moisture. A diainagesystem can be effective in minimizing roadbed swellingif it reduces the availability of moistureforabsorption.

Frost heave, as it is treated here, refers to thelocalized volume changes that occur in the roadbed asmoisture collects, freezes into ice lenses, and producesdistortions on the pavement surface. Like swelling, theeffects of frost heave can be decreased by providingsome type of drainage system. perhaps a more effec-tive measure is to provide a layer of nonfrost-susceptible material thick enough to insulate theroadbed soil from frost penetration. This not onlyprotects against frost heave, but also significantlyreduces or even eliminates the thaw-weakening thatmay occur in the roadbed soil during early spring.

If either swelling or frost heave is tobe considered interms of their effects on serviceability loss and the needfor future overlays, then the following procedureshould be applied. It requires the plot of slrviceabilityloss versus time developed in Section 2.1.4.

The procedure for considering environmentalserviceability loss is similar to the treatment of stageconstruction strategies because of the planned futureneed for rehabilitation. In the stage constructionapproach, an initial PCC slab thickness is selected andthe corresponding performance period (sendce life)determined. An overlay (or series of overlays) whichwill extend the combined performance periods pastthe desired analysis period is then identified. Thedifference in the stage construction approach whenswelling andlor frost heave are considered is that aniterative process is required to determine the length of

Design Requirements

stiffness, and future mechanistic-based proceduresmay reflect strength as well as stiffness in the materialscharacterization procedures. In addition, stabilizedbase materials may be subject to cracking undercertain conditions and the stiffness may not be anindicator for this distress type. It is important to note,that, although resilient modulus can apply to any typeof material, the notation M* as used in this Guideapplies only to the roadbed soil. Different notationsare used to express the moduli for subbase (Err), base(Egs), asphalt concrete (Enc), and portland cementconcrete (Ec).

The procedure for estimating the resilient modulusof a particular pavement material depends on its type.Relatively low stiffness materials, such as natural soils,unbound granular layers, and even stabilized layersand asphalt concrete, should be tested using theresilient modulus test methods (AASHTO T274).Although the testing apparatus for each of these typesof materials is basically the same, there are somedifferences, such as the need for triaxial confinementfor unbound materials.

Alternatively, the bound or higher stiffness materials,such as stabilized bases and asphalt concrete, may betested using the repeated-load indirect tensile test(ASTM D4123). This test still relies on the use ofelectronic gauges to measure small movements of thesample under load, but is less complex and easier torun than the triaxial resilient modulus test.

Because of the small displacements and brittlenature of the stiffest pavement materials, i.e., portlandcement concrete and those base materials stabilizedwith a high cement content, it is difficult to measurethe modulus using the indirect tensile apparatus. Thus,it is recommended that the elastic modulus of suchhigh-stiffness materials be determined according tothe procedure described in ASTM C469.

The elastic modulus for any type of material mayalso be estimated using correlations developed by thestate's department of transportation or by some otherreputable agency. The following is a correlationrecommended by the American Concrete Institute (l)

for normal weight portland cement concrete:

' t" = 57ooo (f ;)o't

where

Ec = PCC elastic modulus (in psi),

II-17

PCC compressive strength (in psi) asdetermined using AASHTO Tzz,Tl4f,or ASTM C39.

2.3.4 PCC Modulus of Rupture

The modulus of rupture (flexural strength) ofportland cement concrete is required only for thedesign of a rigid pavement. The modulus of rupturerequired by the design procedure is the mean valuedetermined after 28 days using third-point loading(AASHTO T97, ASTM C78). If standard agencypractice dictates the use of center-point loading, then acorrelation should be made between the two tests.

Because of the treatment of reliability in this Guide,it is strongly recommended that the normal construc-tion specification for modulus of rupture (flexuralstrength) not be used as input, since it represents avalue below which only a small percent of the dis-tribution may lie. If it is desirable to use the construc-tion specification, then some adjustment should beapplied, based on the standard deviation of modulusof rupture and the percent (PS) of the strengthdistribution that normally falls below the specification:

S'" (mean) = S" + z(SDr)

where

S'" = estimated mean value for PCC modulusof rupture (psi),

Sc = construction specification on concretemodulus of rupture (psi),

SD, = estimated standard deviation of concretemodulus of rupture (psi),

z = standard normal variate:

= 0.841, for PS = 20 percent,*

= 1.037, for PS = 15 percent,

= 1.282, for PS = l0 percent,

= 1.U5, for PS = 5 percent,

= 2.327, for PS = I percent.

'Note: Permissible number of specimens, expressed

as a percentage, that may have strengths less than thespecification value.

11.18

2.3,5 Layer Coeflicients

This section describes a method for estimating theAASHTO structural layer coefficients (ai values)required for standard flexible pavement structuraldesign. A value for this coefficient is assigned to eachlayer material in the pavement structure in order toconvert actual layer thicknesses into structural number(SN). This layer coefficient expresses the empiricalrelationship between SN and thickness and is ameasure of the relative ability of the material tofunction as a structural component of the pavement.The following general equation for structural numberreflects the relative impact of the layer coefficients (a,)and thickness (D,):

sS N =

Z - r l l

I

Although the elastic (resilient) modulus has beenadopted as the standard material quality measure, it isstill necessary to identify (corresponding) layer coef-ficients because of their treatment in the structuralnumberdesign approach. Though there are correlationsavailable to determine the modulus from tests such asthe R-value, the procedure recommended is directmeasurement usi ng- $0S HTO Met hod T 27 4 (subbaseand unbound granular materials) and ASTM D4123for asphalt concrete and other stabilized materials.Research and field studies indicate many factorsinfluence the layer coefficients, thus the agency'sexperience must be included in implementing theresults from the procedures presented. For example,the layer coefficient may vary with thickness, under-lying support, position in the pavement structure, etc.

It should be noted that laboratory resilient modulusvalues can be obtained that are significantly differentfrom what may exist for an in situ condition. Forexample, the presence of a vcly stiff unbound layerover a low stiffness layer may result in decompactionand a corresponding reduction of stiffness. As aguideline for successive layers of uubuuud rnatcrials,the ratio of resilient modulus of the upper layer to thatof the lower layer should not exceed values that resultin tensile stresses in unbound granular layers.

The discussion of how these coefficients are estimatedisseparated into five categories, depending on the typeand function of the layer material. These are asphaltconcrete, granular base, granular subbase, cement-treated, and bituminous base. Other materials such as


lime, lime flyash, and cement flyash are acceptablematerials, and each agency should develop charts.

Asphalt Concrete Surlace Course. Figure 2.sprovides a chart that may be used to estimate thestructural layer coefficient of a dense-graded asphaltconcrete surface course based on its elastic (resilient)modulus (Eec) at 68oF. Caution is recommended formodulus values above 450,000 psi. Although highermodulus asphalt concretes are stiffer and more resistantto bending, they are also more suiceptible to thermaland fatigue cracking.

Gronular Base Loyers. Figure 2.6 provides a chartthat may be used to estimate a structural layercoefficient, &2, from one of four different laboratorytest results on a granular base material, including baseresilient modulus, E"r. The AASHO Road Test basisfor these correlations is:

0 . 1 430,000 psi100 (approx.)85 (approx.)

a 2 =Egs =

CBR =

R-value =

The following relationship may be used in lieu ofFigure 2.6 to estimate the layer coefficient, a,2, for agranular base material from its elastic (resilient)modulus, Er, (s):

az= 0.249(log,oE"r) - 0.977

For aggregate base layers, E* is a function of thestress state (0) within the layer and is normally given bythe relation:

Ess = krek2

where

e = stress state or sum of principal stresseso t + a 2 * t 3 ( P s i ) ,

k,, k, = regressionconstantswhich areafunctionof material type.

Typical values for base materials are:

kt = 3ooo to 8ooo

k2 = o'5 to o'7

\

{ r :

II.20

0.20

0 . 1 8

0 . 1 6

o . 1 4

0 . 1 2

0 . 1 0

0.08

0.06

0.04

0.02

0

c o -C)

60

o 7 0' 6 0

E .'/nO !'v

e 4 09 - -ci 306

= a n

v)

g

oct

O

U'

=!o

(1) Scale der ived by averag ing corre la t ions obta ined f rom l l l ino is .Ql Scale derived by averaging correlat ions obtained from Cali fornia, New Mexico and Wyoming.(3) Scale derived by averaging correlat ions obtained from Texas.Al Scale clerived on NCHRP Orojec.t (3).

Figure 2.6. Variat ion in granular base layer coeff icient (a 2 ) with

various base strength parameters (3l.


Design Requirements

At the AASHO Road Test, modulus values (Ess in psi) for the base were as follows:

Stress State (psi)

Moisture State Equetion 0 = 5 0 = t 0 0 = 2 0 0 = 3 0

II-21

Dry

Damp

Wet

8000e0'6

4000e0.6

3200eo'6

21,012

10,506

8,Q4

31,848

15,924

12,739

48,273 61,569

24,136 30,784

19,309 24,627

Note, E", is a function of not only moisture but alsothe stress state (6). Values for the stress state within the

AsphaltConcrete Thickness (inches)

base course vary with the subgrade modulus andthickness of the surface laye r. Typical values for use indesign are:

Roadbed Soil Resilient Modulus (psi)

3,000 7,500 15,000

Less than 2

2 - 4

4 - 6

Greater than 6

20

l0

5

5

25

l 5

l 0

5

30

20

l 5

5

For intermediate values of roadbed soil resilientmodulus, interpolation can be used.

Each agency is encouraged to develop relationshipsfor their specific base materials (e.9., MR = k,QKz)using AASHTO Method T274;however, in the absenceof this data, values given in Table 2.3 can be used.

Granulor Subbose Layers. Figure 2.7 provides achart that may be used to estimate a structural layercoefficiente ?3, from one of four different laboratoryresults on a granular subbase material, includingsubbase resilient modulus, Err. The AASHO RoadTest basis for these correlations is:

a 3 = 0 . I lEr" = 15,000 psi

CBR = 30 (appox.)R-value = 60 (appox.)

The E* versus a, relationship (s) similar to that forgranular base materials is as follows:

at = 0.227(logro Err) - 0.839

For aggregate subbase layers, E* is affected bystress state (0) in a fashion similar to that for the baselayer. Typical values for k, range from 1500 to 6000,while k, varies from 0.4 to 0.6. Values for the AAHSORoad Test subbase material were (r;):

Stress State (psi)0 = 5 0 = 7 . 5 0 = 1 0

MoistureStete

DevelopedReletionship

Damp

Wet

= 5400 0 0'6

= {6,00 g 0'6

l 4 , l g3

12,082

MR

MR

18,090 21,497

15,410 18 ,312

II-22 Design of Pavement Structures

Table 2.3 Typical values for k, and k2 for unbound base and subbasematerials (Mn = k, 0 k2l.

(al Base

MoistureCondition k r ' k2*

DryDampWet

6,000 - 10,OOO4,OOO - 6,0002,OOO - 4,OOO

0.5 - o.70.5 - 0.70.5 - 0.7

(bl Subbase

DryDampWet

6,000 - 8,OOO4,OOO - 6,0001,500 - 4,OOO

o.4 - 0.6o.4 - 0.60.4 - 0.6

* Range in k., and k, is a function of the material quality.

As with the base layers, each agency is encouragedto develop relationships for their specific materials;however, in lieu of this data, the values in Table 2.3 canbe used.

Stress states (0) which can be used as a guide toselect the rnodulus value for subbase thicknessesbetween 6 and 12 inches are as follows:

its Marshall stability (AASHT0T24',ASTM D I 559).This is not shown in Figure2.9.

2.4 PAVEMENT STRUCTURALCHARACTERISTICS

2,4.1Dreinege

This section describes the selection of inputs to treatthe effects of certain levels of drainage on predictedpavement performance. Guidance is not provided herefor any detailed drainage designs or constructionmethods. Furthermore, criteria on the ability ofvarious drainage methods to remove moisture fromthe pavement are not provided. It is up to the designengineer to identify what level (or quality) of drainageis achieved under a specific set of drainage conditions.Below are the general definitions corresponding todifferent drainage levels from the pavement structure:

Qualityof

Drainage

WaterRemovedWithin

AsphaltConcreteThickness(inches)

less than 22 - 4

greater than 4

StressState(psi)

10.07.55.0

Cement-Treated Bases. Figure 2.8 provides a chartthat may be used to estimate the structural layercoefficient,a2, fora cement-treated base material fromeither its elastic modulus, Er' or, alternatively, its7-day unconfined compressive strength (ASTMDr633).

Bituminous-Treated Bases. Figure 2.9 presents achart that may be used to estimate the structural layercoefficient, ?2, for a bituminous-treated base materialfrom either its elastic modulus, Era, or, alternatively,

ExcellentGoodFairPoorVery Poor

2 hoursdayweekmonth

( w a t e r w i l lnot drain)

Design Requirements

0.20

0 . 1 4

o . 1 2

0 . 1 0

0.08

0.06

11.23

@o-oOo

(tJ

.tto

007050q

30

1 0

20

9 0 -

80

70

60

.9X

.9h

20

1 5( l ) v

EI

- 8 . - E _ _ _ _ ! 45 1 3

1 21 11 0

(11 Scale derived from correlat ions from l l l inois.

Ql Scale derived from correlations obtained from The Asphalt Institute, California, New

Mexico and Wyoming.

(3) Scale derived from correlat ions obtained from Texas.

(41 Scafe derived on NCHRP proiect (3).

Figure 2.7. Variation in granular subbase layer coefficient (a3l with

various subbase strength parameters (31-

q

30

25

- - 1c)

(o

c.9.9

c)()

I

V)

11.24 Design of Pavement Structures

.26

.24

.22

o.20

. 1 8

. 1 6

1 4

0 . ' t 2

0 . 1 0

t\lo

x

r\

o

-q)

(n

o

EC)

-G

f

O

a

(1) Scale der ived by averag ing corre la t ions f rom l l l ino is . Lou is iana and Texas.

l2 l Sca le der ived on NCHRP pro ject (3) .

Figure 2.8. Var iat ion in a for cement- t reated bases with base strength parameter (31.

\,

oo.

rf)

oI

of

Eo

Design Requirements

0.20

0 . 1 4

o . 1 2

0 . 1 0

0.08

0.06

11.23

9 0 -

80

70

60

(11 Scale derived from correlat ions from lf l inois.

l2l Scale derived from correfations obtained from The Asphalt Institute, California, NewMexico and Wyoming.

(3) Scale derived from correlations obtained from Texas.

l4l Scafe derived on NCHRP proiect (3).

Figure 2.7. Variat ion in granular eubbase layer coeff iciont (.g) with

various subbase strength parameters (31.

(vt(g

c.9.9q)o -

(J

6

o

a

' 6o

ooo

of

o

.9x(DL

Fo(Dxo

1007050Q

30

20

1 0q

30

25

1 51 41 31 21 11 0

Design Requirements

0.30

3 0

2 . 5

2 0

0.20

0 . 1 0

(1) Scale der ived by cor re la t ign obta ined f rom l l l ino is .

Ql Scale der ived on NCHRP pro ject (3) .

Figure 2.9. Var iat ion in a, for b i tuminous-treated bases with base strength parameter (3).

II-25

oo-

rr)oF

I

@

=!o

C\{(o

c.9.9oo(-)(]f(J

(n

11.26

For comparison pu{poses, the drainage conditions atthe AASHO Road Test are considered to be fair, i.e.,

free water was removed within I week.

Flexibte Povements. The treatment for the expectedlevel of drainage for a flexible pavement is through theuse of modified layer coefficients (e.9., a highereffective layer coefficient would be used for improveddrainage conditions). The factor for modifying thelayer coefficient is referred to as an m, value and hasbeen integrated into the structural number (SN)

equation along with layer coefficient (a,) and thickness(Di); thus:

SN = utDl + arDrmr+ arDrm,

(The possible effect of drainage on the asphaltconcrete surface course is not considered.) Theconversion of the structural number into actualpavement layer thicknesses is discussed in more detailin Chapter 3.

Table 2.4 presents the recommended m, values as afunction of the quality of drainage and the percent oftime during the year the pavement structure wouldnormally be exposed to moisture levels approachingsaturation. Obviously, the latter is dependent on theaverage yearly rainfall and the prevailing drainageconditions. As a basis for comparison, the m, value for

Table 2.4


conditions at the AASHO Road Test is 1.0, regardlessof the type of material. A discussion of how theserecommended m, values were derived is presented inAppendix DD of Volume 2.

Finally, it is also important to note that these valuesapply only to the effects of drainage on untreated baseand subbase layers. Although improved drainage iscertainly beneficial to stabihzed or treated materials,the effects on performance of flexible pavements arenot as profound as those quantified in Table 2.4.

Rigid Pavemenls. The treatment for the expectedlevel of drainage for a rigid pavement is through theuse of a drainage coefficient, Co, in the performanceequation. (It has an effect similar to that of the loadtransfer coefficient, J.) As a basis for comparison, thevalue for Co for conditions at the AASHO Road Testis 1.0.

Table 2.5 provides the recommended Co values,depending on the quality of drainage and the percentof time during the year the pavement structure wouldnormally be exposed to moisture levels approachingsaturation. As before, the latter is dependent on theaverage yearly rainfall and the prevailing drainageconditions. A discussion of how these recommendedCo values were derived is also presented in AppendixDD of Volume 2.

Recommended m, values for modifying structural layer coefficients of untreated base andsubbase materials in flexible pavements.

Ouality ofDralnage

Percent of Time Pavcment Structure is Exposedto Molsture Levels Approaching Saturation

Less Than1 % 1 - 5 % 5 - 2 5 %

Greater Than25%

Excellent

Good

Fair

Poor

Very Poor

1 . 4 0 - 1 . 3 5

1 . 3 5 - 1 . 2 5

1 . 2 5 - 1 . 1 5

1 . 1 5 - 1 . O 5

1.O5 - O.95

1 . 3 5 - 1 . 3 0

1 . 2 5 - 1 . 1 5

1 . 1 5 - 1 . O 5

1.O5 - O.80

o.95 - 0.75

1 .30 - 1 .20

1 . 1 5 - 1 . O O

1.OO - 0.80

o.80 - 0.60

o.75 - O.40

1 .20

1.OO

o.80

o.60

0.40

Design Requiements II-27

Table 2.5. Recomm6ndsd values of drainage coefficient' Co, {or rigid pavemsnt dosign'

Ouali ty ofDra inage

Percent of Time Pavement Structure is Exposedto Moisture Levels Approaching Saturation

Less Than1o/o 1 - 5 % 5 - 2 5 %

Greater Than25o/o

Excel lent

Good

Fatr

Poor

Very Poor

1 . 2 5 - 1 . 2 0

1 . 2 0 - 1 . 1 5

1 . 1 5 - 1 . 1 0

1 . 1 0 - 1 . o o

1.OO - O.90

1 . 2 0 - 1 . 1 5

1 . 1 5 - 1 . 1 0

1 . 1 0 - 1 . O O

1.OO - 0.90

o.90 - o.80

1 . 1 5 - 1 . 1 0

1 . 1 0 - 1 . O O

1.OO - O.90

0.90 - 0.80

o.80 - o.70

1 . 1 0

1.00

o.90

o.80

o.70

2.4.2 Load Transfer

The load transfer coefficient, J, is a factor used in

rigid pavement design to account for the ability of a

concrete pavement structure to transfer (distribute)

load across discontinuities, such as joints or cracks.

Load transfer devices, ag9regate interlock, and thepresence of tied concrete shoulders all have an effect

on this value. Generally, the J-value for a given set of

conditions (e .g., jointed concrete pavement with tied

shoulders) increases as traffic loads increase since

aggregate load transfer decreases with load repetitions.

Table 2.6 establishes ranges of load transfer coefficients

for different conditions developed from experience

and mechanistic stress analysis. As a general guide for

the range of J-values, higher J's should be used withlow k-values, high thermal coefficients, and largevariations of temperature. (The development of the

J-factor terms is provided in Appendix KK of Volume

2.) Each agency should, however. develop criteria for

their own aggrcgates. climatic conditions. etc.

If dowels are used, the size and spacing should be

determined by the local agency's procedures and/or

experience. As a general guideline, the dowel diameter

should be equal to the slab thickness multiplied by Vtinch (e.g., for a l0-inch pavement, the diameter is l '%

inch. The dowel spacing and length are normally l2

inches and l8 inches, respectively.

Jointed Povemenfs. The value of J recommended

for a plain jointed pavement (JCP) orjointed reinforced

concrete pavement (JRCP) with some type of load

transfer device (such as dowel bars) at the joints is 3.2("protected corner" condition at the AASHO Road

Test). This value is indicative of the load transfer ofjointed pavements without tied concrete shoulders.

Forjointed pavements without load transfer devices

at the joints, a J-value of 3.8 to 4.4 is recommended.(This basically accounts for the higher bending stresses

that develop in undowelled pavements, but also in-

cludes some consideration of the increased potential

for faulting.) If the concrete has a high thermal

coefficient, then the value of J should be increased. On

the other hand, if few heavy trucks are anticipated

such as a low-volume road, the J-value may be lowered

since the loss of aggregate interlock will be less. Part I

of this Guide provides some other general criteria for

the consideration andlor design of expansion joints,

contraction joints, longitudinal joints, load transfer

devices, and tie bars in jointed pavements.

Continuously Reintorced Psvemenfs. The value of

J recommended for continuously reinforctcd c:ont:rete

pavements (CRCP) without tied concrete shoulders is

between 2.9 to 3.2, depending on the capability of

aggregate interlock (at future transverse cracks) to

transfer load. In the past, a commonly used J-value for

CRCP was 3.2, but with better design for crack width

control each agency should develop criteria based on

local aggregates and temperature ranges.

Tied Shoulders or lTidened Outside Lanes. One of

the major advantages of using tied PCC shoulders (or

widened outside lanes) is the reduction of slab stress

and increased service life they provide. To account for

this, significantly lower J-values may be used for the

design of both jointed and continuous pavements.

11.28 Design of Pavement Structures

Recommended load transfer coefficient for variouspavement types and design conditions.

Table 2.6.

Shou lde r Asphalt Tied P.C.C.

Load TransferDevices Yes N o Yes No

Pavement Type

1 . P la i n Jo in tedand

Join ted Reinforced3.2

2 , CRCP 2.9 - 3 .2 N/A 2.3 - 2 .9 N/A

For continuously reinforced concrete pavementswith tied concrete shoulders (the minimum bar sizeand maximum tie bar spacing should be the same asthat for tie bars between lanes), the range of J isbetween 2.3 and 2.9, with a recommended value of 2.6.This value is considerably lower than that for thedesign of concrete pavements without tied shouldersbecause of the significantly increased load distributioncapability of concrete pavements with tied shoulders.

v, For jointed concrete pavements with dowels andtied shoulders, the value of J should be between 2.5and 3.1 based on the agency's experience. The lowerJ-value for tied shoulders assumes traffic is notpermitted to run on the shoulder.

Note: Experience has shown that a concrete shoulderof 3 feet or greater may be considered a tied shoulder.Pavements with monolrthic or tied curb and gutterthat provides additional stiffness and keeps trafficaway from the edge may be treated as a tied shoulder.

2.4.3 Loss of Support

This factor, LS, is included in the design of rigidpavements to account for the potential loss of supportarising from subbase erosion and/or differentialvertical soil movements. It is treated in the actualdesign procedure (discussed in Chapter 3) by dimin-ishing the effective or composite k-value based on thesize of the void that may develop beneath the slab.

Table 2.7 provides some suggested ranges of LSdepending on the type of material (specifically itsstiffness or elastic modulus). Obviously, if varioustypes of base or subbase are to be considered fordesign, then the corresponding values of LS should bedetermined for each type. A discussion of how the lossof support factor was derived is present in AppendixLL of Volume 2 of this Guide.

The LS factor should also be considered in terms ofdifferential vertical soil movements that may result invoids beneath the pavement. Thus, even though anonerosive subbase is used, a void may still develop,thus reducing pavement life. Generally, for activeswelling clays or excessive frost heave, LS values of 2.0to 3.0 may be considered. Each agency's experience inthis area should, however, be the key element in thesclection of an appropriate LS value. Examination ofthe effect of LS ou reducing thc cffactive k value of thcroadbed soil (see Figure 3.6) may also be helpful inselecting an appropriate value.

2.5 REINFORCEMENT VARIABLES

Because of the difference in the reinforcementdesign procedures between jointed and continuouspavements, the design requirements for each areseparated into two sections. Information is also pro-vided here for the design of prestressed concretepavement. In addition to dimensions, considerationshould be given to corrosion resistance of reinforce-ment, especially in areas where pavements are exposedto variable moisture contents and salt applications.

Highway Pavement Structural Design II-37

Minimum Thickness (inches)

Traffic, ESALs Asphelt Concrete Aggregate Base

Less than 50.000

50,001 - 150,000150,001 - 500,000

500,001 - 2,000,0002,000,001 - 7,000,000

Greater than 7,000,000

1.0 (or surfacetreatment)

2.02.53.03 .54.0

4

44666

\,1,

Because such minimums depend somewhat on localpractices and conditions, individual design agencies

may find it desirable to modify the above minimum

thicknesses for their own use.

Individual agencies should also establish the effectivethicknesses and layer coefficients of both single anddouble surface treatments. The thickness of the surfacetreatment layer may be neglectible in computing SN,but its effect on the base and subbase properties maybe large due to reductions in surface water entry.

3.f.5 Layered Design Analysis

It should be recognized that, for flexible pavements,

the structure is a layered system and should bedesigned accordingly. The structure should be designedin accordance with the principles shown in Figure 3.2.First, the structural number required over the roadbed

soil should be computed. In the same w&y, thestructural number required over the subbase layer andthe base layer should also be computed, using theapplicable strength values for each. By working withdifferences between the computed structural numbersrequired over each layer, the maximum allowablethickness of any given layer can be computed. For

example, the maximum allowable structural numbcrfor the subbase material would be equal to thestructural number required over the subbase subtractedfrom the structural number required over the roadbedsoil. In a like manner, the structural numbers of the

other layers may be computed. The thicknesses for the

respective layers may then be determined as indicatedon Figure 3.2.

It should be recognized that this procedure shouldnot be applied to determine the SN required abovesubbase or base materials having a modulus greater

than 40,000 psi. For such cases, layer thicknesses ofmaterials above the "high" modulus layer should be

established based on cost effectiveness and minimumpractical thickness considerations.

3.2 RIGID PAVEMENT DESIGN

This section describes the design for portland ce-ment concrete pavements, including plain jointed

(JCP), jointed reinforced (JRCP), and continuouslyreinforced (CRCP). As in the design for flexiblepavements, it is assumed that these pavements willcarry traffic levels in excess of 50,000 l8-kip ESALover the performance period. An example of theapplication of this rigid paveme nt design procedure ispresented in Appendix L.

The AASHTO design procedure is based on theAASHO Road Test pavement performance algo-rithm. Inherent in the use of the procedure is the use ofdowels at transverse joints. Hence, joint faulting wasnot a distress manifestation at the Road Test. If thedesigner wishes to consider nondowelled joints, hemay develop an appropriate J-factor (see Section2.4.2, "Load Transfer') or check his design withanother agency's irocedure, such as the PCA pro-

cedure (q).

3.z.LDevelop Effective Modulus of Subgrade Reaction

Before the design chart for determining design slabthickness can be applied, it is necessary to estimate thepossible levels of slab support that can be provided.This is accomplished using Table 3.2 and Figures 3.3,3.4, 3.5, and 3.6 to develop an effective modulus ofsubgrade reaction, k. An example of this process isdemonstrated in Table 3.3.

Since the effective k-value is dependent upon severaldifferent factors besides the roadbed soil resilientmodulus, the first step is to identify the combinations(or levels) that are to be considered and enter them inthe heading of Table 3.2.

(l) Subbase types - Different types of subbase

have different strengths or modulus values.

The consideration of a subbase type in

estimating an effective k-value provides a

Highway Pavement Structural Design

basis for evaluating its cost-effectiveness aspart of the design process.

(2) Subbase thicknesses (inches) - Potentialdesign thicknesses for each subbase typeshould also be identified, so that its cost-effectiveness may be considered.

(3) Loss of support, LS - This factor, quanti-

fied in Section 2.4.3, is used to correct theeffective k-value based on potential erosionof the subbase material.

:

(4) Depth to rigid foundation (feet) - If bed-rock lies within l0 feet of the surface of thesubgrade for any significant length along theproject, its effect on the overall k-value andthe design slab thickness for that segmentshould be considered.

For each combination of these factors that is to beevaluated, it is necessary to prepare a seperate tableand develop a corresponding effective modulus ofsubgrade reaction.

The second step of the process is to identify the

seasonal roadbed soil resilient modulus values (from

Section 2.3.1) and enter them in Column 2 of each

table. As before, if the length of the smallest season is

one-half month, then all seasons must be defined in

terms of consecutive half-month time intervals in the

table. (The same seasonal roadbed soil resilient

modulus values used for the example in Section 2.3.1

are used in the example presented in Table 3.3.)

The third step in estimating the effective k-value is

to assign subbase elastic (resilient) modulus (Esn)

values for each season. These values, which were

discussed in Section 2.3.3, should be entered in

Column 3 of Table3.2and should correspond to those

for the seasons used to develop the roadbed soil

resilient modulus values. For those types of subbase

material which are insensitive to season (e.9., cement-

treated material), a constant value of subbase modulus

may be assigned for each season. For those unbound

materials which are sensitive to season but were not

tested for the extreme conditions, values for Er" of

50,000 psi and 15,000 psi may be used for the frozen

and spring thaw periods, respectively. For unboundmaterials, the ratio of the subbase to the roadbed soil

resilient modulus should not exceed 4 to prevent an

artificial condition.

The fourth step is to estimate the composite modulus

of subgrade reaction for each season, assuming a semi-

II-45

infinite subgrade depth (i.e., depth to bedrock greater

than l0 feet) and enter in Column 4. This is accom-plished with the aid of Figure 3.3. Note that thestarting point in this chart is subbase thickness, DSB. Ifthe slab is placed directly on the subgrade (i.e., nosubbase), the composite modulus of subgrade reactionis defined using the following theoretical relationshipbetween k-values from a plate bearing test and elasticmodulus of the roadbed soil:

k = M* / 19 .4

Note: The development of this relationship is de-scribed as part of Volume 2, Appendix HH.

The fifth step is to develop a k-value which includes

the effect of a rigid foundation near the surface. This

step should be disregarded if the depth to a rigidfoundation is greater than l0 feet. Figure 3.4 provides

the chart that may be used to estimate this modified

k-value for each season. It considers roadbed soil

resilient modulus and composite modulus of subgradereaction, as well as the depth to the rigid foundation.The values for each modified k-value should sub-sequently be recorded in Column 5 of Table 3.2.

The sixth step in the process is to estimate thethickness of the slab that will be required, and then use

Figure 3.5 to determine the relative damage, ur, in eachseason and enter them in Column 6 of Table 3.2.

The seventh step is to add all the u, values (Column

6) and divide the total by the number of seasonal

increments (12 or 24) to determine the average relative

damage, ur. The effective modulus of subgradereaction, then, is the value corresponding to the

average relative damage (and projected slab thickness)in Figure 3.5.

The eighth and final step in the process is to adjustthe el'lective modulus of subgrade reaction to accountfor the potential loss of support arising from subbase

erosion. Figure 3.6 provides the chart for correctingthe effective modulus of subgrade reaction based on

the loss of support factor, LS, determined in Section

2.4.3. Space is provided in Table 3.2 to record this final

design k-value.

3.2.2 Determine Required Sleb Thickness

Figure 3.7 (in 2 segments) presents the nomograph

used for determining the slab thickness for each

effective k-value identified in the previous section. The

designer may then select the optimum combination of

slab and subbase thicknesses based on economics and

Design of Pavement Structures11.46

o3 ^b c v-i a, -

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tr, c\r 3 tt, EctcF. ; E

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11.47H ighway Pavement Structural D esign

v,t!

too

J

=a0a).9Lo(tcot.Do

Estimoted Totol 18- kiPLood (ESAL) A

r@ 50 lo

I Single Axle(mil l ions)

to .5

NOTE, Applicolion ol reliobililyin itris chorl requireslhe use ol meon voluesfor oll lhe inPul vorlobles.

Figure.3.7. Derign chart for rigid pavements based on using mean values for

oach inPut variable (Segment 21.

Rel iobi l i ty , R (o/o)